There are various forms of automatic injection device which when operated cause the needle of a syringe to be moved forwardly so that it projects from a protective housing prior to actuation of the syringe to express a dose of liquid through the needle. It is important to try to ensure that the syringe is moved bodily forward to expose the needle before the liquid charge is pressurised so that dribbling from the needle does not occur before the actual injection takes place. It is an object of this invention to provide a mechanism which operates in this desired manner.